An experimental and theoretical investigation on the swelling of N-isopropyl acrylamide based ionic hydrogels in aqueous solutions of (sodium chloride or di-sodium hydrogen phosphate)

doi:10.1016/j.fluid.2007.02.010

Fluid Phase Equilibria

Volume 254, Issues 1–2, 15 June 2007, Pages 1-10

https://doi.org/10.1016/j.fluid.2007.02.010 Get rights and content

Abstract

Hydrogels are three-dimensional polymeric networks of hydrophilic monomers. When at least some of the monomers are electrolytes, such networks are called polyelectrolyte hydrogels. This paper reports new experimental results for the swelling equilibrium of polyelectrolyte hydrogels consisting of the neutral monomer N-isopropyl acrylamide and the ionic comonomer sodium methacrylate in aqueous solutions of a single salt (either sodium chloride or di-sodium hydrogen phosphate) at 298 K. The effect of the salt on the swelling equilibrium of such hydrogels is described by a thermodynamic model which combines an expression for the Gibbs energy of a liquid phase with an equation for the Helmholtz energy of an elastic network.

Introduction

Hydrogels are three-dimensional networks composed of crosslinked hydrophilic polymers. They might reveal interesting phase equilibrium phenomena, e.g. a phase transition when some external environmental conditions such as temperature, solvent composition or electromagnetic stimulus are slightly changed [1], [2], [3]. The swelling behaviour can be adjusted by varying the composition of the hydrogel. N-Isopropyl acrylamide (IPAAm) is one of the most important monomers for the production of synthetic hydrogels. Synthetic hydrogels are used and discussed for various applications, e.g. for controlled drug delivery systems, as absorbing agents, for biosensors and dental materials [4], [5], [6].

This paper extends some earlier work on the phase equilibrium of non-ionic IPAAm hydrogels in aqueous electrolyte solutions [7] to the swelling behaviour of polyelectrolyte hydrogels based on IPAAm and the ionic comonomer sodium methacrylate (NaMA) in aqueous solutions of a single salt (either sodium chloride (NaCl) or di-sodium hydrogen phosphate (Na₂HPO₄)) at 298 K. New experimental results for the swelling equilibrium of some hydrogels of different composition are presented and a thermodynamic framework is presented to describe such phase equilibria and to predict the influence of the parameters used to characterize the hydrogels on the swelling equilibrium.

Section snippets

Sample preparation and characterization of hydrogels

A detailed description of the procedure which was applied to produce the hydrogels is available from previous publications [8], [9]. Therefore, only an outline is given here. The hydrogels were prepared in aqueous solutions at 25 °C by free radical polymerization of N-isopropyl acrylamide with sodium methacrylate, using N,N′-methylene bisacrylamide (MBA) as crosslinker and small (and even) amounts of ammonium peroxodisulfate (APS) and sodium disulfite (NaDS) which were used as reaction initiator

Experimental results

The characteristic parameters of the investigated hydrogels are:

•
total mass fraction of polymerizable material: ξ_gel (g/g) = 0.08 and 0.10,
•
mole fraction of the crosslinking agent MBA: y_cr (mol/mol) = 0.01 and 0.015,
•
mole fraction of the ionic comonomer NaMA: y_co (mol/mol) = 0.01, 0.02 and 0.03, respectively, and
•
mass fraction of the initiator pair ξ_S (g/g) = 1.81 × 10⁻⁴.

The experimental results for the degree of swelling are given in Table 1, Table 2 (for the swelling in aqueous solutions of NaCl), and in

Phase equilibrium condition of gel systems

In the present work, the hydrogel phase is treated as a liquid mixture, where the polymer network is represented by non-crosslinked chains (i.e., the network is virtually cut off at all crosslinking positions) and all contributions to the Gibbs energy which are due to crosslinking and extension of the polymer chains are summarized in an expression for the Helmholtz energy A^m of an elastic network. Following Maurer and Prausnitz [10] the equilibrium state of a gel containing system is determined

Correlation and prediction of the swelling equilibrium of IPAAm/NaMA hydrogels in (water + sodium chloride)

For the calculation of the swelling equilibrium of non-ionic IPAAm-gels in aqueous solutions of NaCl the following assumption/approximations are used:

•
NaCl is completely dissociated. The interactions parameters $A_{Na, Cl}^{(0)}$ , $A_{Na, Cl}^{(1)}$ and $B_{N a^{+}, N a^{+}, C l^{-}}$ were taken from Pitzer [14].
•
The parameters for interactions between groups of IPAAm ( $A_{IPAAm, IPAAm}^{(0)}$ and $A_{IPAAm, IPAAm}^{(1)}$ ) and between IPAAm-groups and sodium ions ( $A_{IPAAm, N a^{+}}^{(0)}$ ) as well as the network parameter C_nonion (for a hydrogel

Conclusion

The swelling equilibrium of IPAAm/NaMA hydrogels in aqueous solutions of the single salts NaCl and Na₂HPO₄ was investigated experimentally at 298 K. The new experimental results were described applying a thermodynamic model. That model combines an extension of Pitzer's model for aqueous electrolyte solutions for the excess Gibbs energy of an aqueous phase with an extension of the phantom network theory. That extension was necessary to account for the large volume changes observed when ionic

Acknowledgements

GM gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG), Bad Godesberg, Germany. XX wants to thank Konrad-Adenauer-Stiftung, Bonn-Bad Godesberg, Germany for providing a scholarship.

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Experimental results are reported for the swelling equilibrium of some ionic hydrogels consisting of N-isopropyl acrylamide (N-IPAAm) and one or two comonomers – the anionic monomer 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) and/or the cationic monomer 3-(methacrylamido)propyl trimethylammoniumchloride (MAPTAC) – in water and in aqueous solutions of sodium chloride at 298 K. The degree of swelling of such ionic hydrogels increases with the net charge (i.e., the difference in the mole fractions of anionic and cationic comonomers) as well as with the charge density (i.e. the mole fraction of a single comonomer). The mole fraction of a single comonomer was varied between 0 and 0.05. The (absolute) difference between the mole fractions of the anionic and cationic comonomers was varied between 0 and 0.03. Two characteristic transitions in the degree of swelling of the hydrogels were observed when sodium chloride is added to the coexisting liquid phase. The first transition depicts the conversion of the ionic comonomer into the neutral state. In that transition the degree of swelling of a charged ionic hydrogel reduces to the degree of swelling of a nonionic hydrogel. For the investigated hydrogels, that transition occurs when the mass fraction of NaCl in the coexisting aqueous phase is increased beyond approximately 5 × 10⁻⁵. The second transition occurs at much higher sodium chloride mass fractions (around 5 × 10⁻³). In that transition the hydrogel collapses, i.e., that transition resembles the phase transition observed when non-ionic N-IPAAm hydrogels are dissolved in aqueous solutions of sodium chloride.
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Hydrogels are crosslinked polymers of hydrophilic monomers. Hydrogels can swell and shrink in aqueous solutions. The swelling behavior of hydrogels and the encountered phase behavior are of interest in many areas, e.g., in biotechnology, membrane science and controlled drug release. This contribution presents the criteria for such phase equilibria and a previously developed thermodynamic model for correlating/predicting the swelling and shrinking of hydrogels. The application of the method is demonstrated by describing the swelling equilibrium of some synthetic, non-ionic N-isopropyl acrylamide (N-IPAAm) hydrogels in aqueous solutions of sodium chloride at 298 K. Furthermore, new experimental results are presented for the degree of swelling of synthetic hydrogels that contain – besides the non-ionic monomer N-IPAAm – either a combination of a cationic comonomer (here, N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA)) and an anionic comonomer (here, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)) or a zwitterionic comonomer (here, [3-(methacryloylamino)propyl]dimethyl(3-sulfopropyl)ammonium hydroxide inner salt (MPSA)). These gels were equilibrated with aqueous solutions of sodium chloride at 298 K.

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An experimental and theoretical investigation on the swelling of N-isopropyl acrylamide based ionic hydrogels in aqueous solutions of (sodium chloride or di-sodium hydrogen phosphate)

Abstract

Introduction

Section snippets

Sample preparation and characterization of hydrogels

Experimental results

Phase equilibrium condition of gel systems

Correlation and prediction of the swelling equilibrium of IPAAm/NaMA hydrogels in (water + sodium chloride)

Conclusion

Acknowledgements

Eur. J. Pharm. Biopharm.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Adv. Polym. Sci.

AIChE J.